In the extrusion of thermoplastic polymers the extrusion apparatus can develop hotspots which may cause scorching of the polymer. The scorching is a problem when the extruded polymer is to be used for its light transmission and clarity.
In the extrusion of ionomers, some of the ionomers are neutralized and/or cross-linked with alkali metal hydroxides during the extrusion process. In some instances there is incomplete dissolution of the additive in the polymer while in the extruder so as to result in the formation of haze in the extruded product. This can cause a problem for polymers which are extruded into sheet form and rely on clarity.
Such sheets are used for laminating for windshield and security glass. It is desirable that the thermoplastic polymers which are used for glazing be haze free and have good optical transmission.
With reference to ionomers, the neutralization or conversion of ethylene/acrylic acid and ethylene/methacrylic acid or methacrylic copolymers to their salts (sodium, potassium, lithium, magnesium or zinc) converts the un-neutralized copolymer ionomer from a tacky polymer with a high melt index and low impact resistance to a relative stiff thermoplastic with high impact resistance. Usually this neutralization or conversion of the free acid groups is converted from 20 to 75% with the preferable range of acid group conversion being 25-50% depending on the particular salt and copolymer involved. This conversion to the partially neutralized or even fully neutralized ionomer is critical to achieving the high impact laminating resins used for laminating glass such as Surlyn® to produce transparent armor, hurricane resistant windows and auto windshields. Absolute optical clarity is the goal for these laminating resins which are used in these optically critical applications such as windshields along with the maximum adhesion possible after laminating to glass to form the laminated product.
U.S. Pat. No. 3,404,134 to Rees, which is herein incorporated by reference, discloses un-neutralized ionomer copolymers that have been cross-linked with up to 73.6% by weight diamines and polyamines in a reactor which were not subject to extrusion. These ionomers were neutralized by alkaline earth and alkali metal cations. The cross-linking and neutralization can be practiced in an extruder according to the invention and provide an improved ionomer.
The preferred neutralizing agent is usually sodium hydroxide, lithium hydroxide or sodium carbonate for the preferred conversion to the sodium or lithium salt of the copolymer. This is accomplished by adding the sodium hydroxide or lithium hydroxide to the polymer pellets as a solid as it is fed into the extruder and the resulting heating and mixing is carried on in the extruder to produce strands of extruded partially neutralized ionomer which is chopped into pellet. This pellet material has a measured transmission in the range of 92-96% which is lower than the 98-100% which is desired. This pellet is then added to another extruder or re-extruded into sheet for laminating. This sheet material after the two extrusions has improved its transmission to about 96-98% which is the accepted maximum transmission by this procedure. The neutralization can actually be performed in the initial polymerization reactor by the careful addition of the neutralizing agent to the aqueous slurry of the freshly produced polymer powder still in the polymerization reactor and this material is stirred and heated until it has reacted. The partially neutralized ionomer powder is washed and dried and then extruded into pellet and then subsequently extruded again into optically clear tough laminating plastic sheet. This laminating sheet after lamination to glass produces a laminate with a transmission of 97-99% which is the highest in the industry and also has a very good impact resistance with a ball drop test. In the test, a 5 lb. steel ball is dropped from 28 inches to determine penetration when dropping.